Problem: Solve for $x$ : $-13 = -10 + x$
Answer: Add $10$ to both sides: $ \begin{eqnarray} \\ \hphantom{-13 {+ 10}} &\hphantom{=}& \hphantom{x} \\ -13 &=& -10 + x \\ \\ {+10} && {+10} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -13 {+ 10} &=& x \\ \end{eqnarray} $ Simplify. $ x = -3$